1. Technical Field
The present invention relates to an image forming apparatus such as a printer and a copy machine employing an electrophotographic system or the like, or a facsimile, and more particularly to an image forming apparatus which, after temporarily transferring a toner image formed on an image carrier onto an intermediate transfer belt, forms an image by transferring the toner image onto a transfer medium.
2. Related Art
Among image forming apparatus of this kind such as a printer and a copy machine employing an electrophotographic system or the like, or a facsimile, as an image forming apparatus capable of forming a full color image, various so-called multipath (4 cycle) type image forming apparatus configured as follows have been previously proposed and commercialized. The configuration is such as to include a single photoreceptor drum, to form in order a toner image for each of the colors yellow (Y), magenta (M), cyan (C) and black (K) on the photoreceptor drum then, after primarily transferring the toner image for each of the colors yellow (Y), magenta (M), cyan (C) and black (K) formed in order on the photoreceptor drum in a mutually superimposed condition onto an intermediate transfer belt, to form a full color image by secondarily transferring from the intermediate transfer belt onto a paper which forms a recording medium.
This kind of multipath (4 cycle) type image forming apparatus employs a synchronization configuration wherein a peripheral length of the intermediate transfer belt is set to be an integral multiple of the peripheral length of a drive roller which drives the intermediate transfer belt, thereby reducing the incidence of color drift due to a frequency fluctuation of the drive roller caused by a bias or the like.
However, even an image forming apparatus employing the synchronization configuration has the following problems which cause color irregularity and other reductions in image quality. That is, an error exists in tolerance and the like in peripheral length of the intermediate transfer belt, and the peripheral length of the intermediate transfer belt varies (extends and contracts) due to environmental changes in temperature, humidity and the like. Furthermore, an error occurs in the synchronization configuration of the peripheral lengths of the intermediate transfer belt and the drive roller due to a load fluctuation when the cleaner which cleans the surface of the intermediate transfer belt comes into and out of contact therewith.
Also, in the case of photographic image quality, as a granularity of a person's “flesh color” is important, yellow, magenta and cyan are staggered by 30 degrees, but this unit that black (K), which has a high visible sensitivity as color irregularity, has the same or similar screen angle (15 degrees or less) as one of the colors yellow, magenta and cyan. As a result, although it is not inevitable that the screen angle of the black image is absolutely identical to the screen angle of another color, a problem exists wherein in the event that setting is done in such a way that the black image deviates by 15 degrees between the images of the other two colors, two kinds of interference are conspicuous, meaning that the moiré is even more conspicuous than in the case of approximately identical angles.
Furthermore, in the case of a digital type screen, as it is not possible to make the line frequency of each color identical and deviate neatly by, for example, 15 degrees or 30 degrees, there is a slight deviation of angle (for example 13 degrees or 27 degrees), meaning that the appearance of the moiré changes depending on the line frequency and resolution. As a result, when looking for an appropriate place while evaluating granularity and the like within a range of 0 to 15 degrees, considering the image quality design parameters, development, transfer and the like, it is preferable to set identical angles for the screen angles of 2 colors, with this setting becoming a necessary condition for the maintenance of photographic image quality. As a result, even though it is becoming extremely important to prevent the occurrence of a large color irregularity between two colors with an equivalent or approximately equivalent screen angle, a problem exists according to the aforementioned related arts wherein it is unable to prevent the occurrence of a large color irregularity between two colors with an equivalent or approximately equivalent screen angle.